This chapter explores fan fiction, defined as stories that reimagine or reinterpret existing stories, characters and universes found in other texts and media, and the informal language learning that reading and writing fan fiction supports. It begins with an overview of fans, fandom, and various fan practices found in online affinity spaces, including both affirmational practices (e.g. the consumption of media, discussion, debating) and transformative practices (e.g. fan art, amateur translations, spoiling). While many of these fan practices have also been found to support informal language learning or the development of other skills and knowledge, it is fan fiction that has received the greatest attention both in popular culture and in educational research. This chapter then offers a crash course overview of different genres of fan fiction and before examining and discussing research which has looked at the informal language learning that occurs through the reading and writing of fan fiction.